1. Technical Field
The present invention relates to a receiver, an integrated circuit and a receiving method for use in digital terrestrial broadcasting based on OFDM (Orthogonal Frequency Division Multiplex) method.
2. Background Art
OFDM method is applied to digital terrestrial broadcasting in Japan and Europe.
In ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) system and DVB-T (Digital Video Broadcasting-Terrestrial) system, pilot signals, whose amplitude and phase are known, are scattered in a frequency domain in sub-carriers. Such pilot signals are called Scattered Pilot Signals (hereinafter called the “SP signals”).
The following describes the arrangement of the SP signals, with reference to FIG. 20. FIG. 20 shows the arrangement of the SP signals in an OFDM signal. Each SP signal is not transmitted by each sub-carrier, but arranged, in the frequency axis direction and the time axis direction, at a position where a carrier number k in a segment satisfies k=3(n mod 4)+12p, where mod is a modulus operator, p is an integer, and n is a symbol number. In other words, the SP signals are arranged in cycles of twelve sub-carriers in the frequency axis direction as FIG. 20 shows. The SP signals are also repeated in cycles of four symbols in the time axis direction. Each SP signal is shifted by three carriers as the symbol number increases by one. Here, note that Continuous Pilot Signals (hereinafter called the “CP signals”) and control information signals are arranged at predetermined sub-carrier positions in the OFDM signal, and information transmission signals are arranged at the other positions.
Each SP signal is transmitted after being modulated into a binary signal by the transmitter, based on a predetermined pattern determined according to the sub-carrier position of the SP signal. The receiver adjusts the phase of the SP signal, and performs interpolation in the frequency axis direction (hereinafter called the “frequency axis interpolation”) and interpolation in the time axis direction (Hereinafter called the “time axis interpolation”) to estimate channel characteristics. Then, the receiver equalizes the received signals based on the estimation.
The time axis interpolation above is described in Patent Document 1 (Japanese Patent Publication No. 3027362) and Patent Document 2 (Japanese Patent Publication No. 3084368) for instance, which are explained next with reference to FIG. 21 and FIG. 22. FIG. 21 is a drawing for explaining the time axis interpolation, and FIG. 22 is a drawing which shows positions of signals whose channel characteristics are interpolated by the time axis interpolation. Note that FIG. 21 only focuses on particular sub-carriers that carry the SP signals.
As FIG. 21 (a) shows, in the time axis interpolation described in the Patent Document 1, the channel characteristics of each SP signal position are kept until next SP signal in the time axis direction. In other words, the same channel characteristics as the channel characteristics of the previous symbol are used for interpolating a symbol whose carrier number is k=3(n mod 4)+12p and by which the SP signal is not carried. This means that the same channel characteristics are used for three symbols at a maximum.
As FIG. 21 (b) shows, in the time axis interpolation described in Patent Document 2, the channel characteristics of the carrier positions are liner-interpolated among symbols, using the channel characteristics of each SP signal position. In circumstances where the channel characteristics vary, this time axis interpolation can more accurately estimate the channel characteristics compared to the time axis interpolation described in Patent Document 1.
In the manner described above, the interpolation is performed in the time axis direction to interpolate the channel characteristics of each signal position shown in FIG. 22.
Patent Document 3 (Japanese Laid-open Patent Application Publication No. 2004-282613) describes, although not in detail, switching between the interpolation processing of Patent Document 1 or Patent Document 2 and interpolation processing that performs filtering in the time axis direction in accordance with the receiving condition. If the filtering is performed in the time axis direction, the interpolation accuracy is expected to be improved compared to the Patent Document 1 and Patent Document 2.
In the case of performing the above-described time axis interpolation, the SP signal is arranged in cycles of four symbols. This means that the SP signal appears once in every four symbols in the time axis direction. The sampling rate in the time axis direction is fs/4, where the inverse of the symbol length is fs. Therefore, according to the sampling theorem, the Doppler frequency up to fs/8 is manageable. For instance, in the case where the guard interval parameter (the proportion of the guard interval length to the effective symbol length) is ⅛ in the mode 3 of the ISDB-T system, the effective symbol length is 1008 μs, the guard interval length is 126 μs, and the symbol length is 1134 μs. Therefore, the fs is 881 Hz (= 1/1134 μs), and the equalization up to the Doppler frequency at 110 Hz is possible in principle.